1. Field of the Invention
The present invention relates to a control device for an air-fuel ratio sensor, and more particularly to a control device for an air-fuel ratio sensor which detects an impedance of an air-fuel ratio sensor element, such as an oxygen concentration detecting element, for accurately and quickly detecting an air-fuel ratio of exhaust gas from an internal combustion engine, the control device detecting a failure and activating a condition of the air-fuel ratio sensor based on the detected impedance and accurately calculating an air-fuel ratio from an output of the air-fuel ratio sensor.
2. Description of the Related Art
In recent years, air-fuel ratio control has been performed using an air-fuel ratio sensor and catalyst disposed in an emission system of the engine with feedback control being carried out so that an air-fuel ratio detected by the air-fuel ratio sensor becomes a target air-fuel ratio, for example, a stoichiometric air-fuel ratio, in order to maximize purification of harmful components (hydrocarbon HC, carbon monoxide CO, nitrogen oxides No.sub.x and the like) in exhaust gas via catalysts. An oxygen concentration detecting element of limit current type outputting a limit current in corresponding to the concentration of oxygen contained in the exhaust gas emitted from the engine has been used for this purpose. The limit current type oxygen concentration detecting element has been used for detecting an air-fuel ratio of exhaust gas from the engine linearly according to the concentration of oxygen and is useful for improving air-fuel ratio control accuracy and for controlling an exhaust gas air-fuel ratio of the engine to a target air-fuel ratio in an interval from a rich or theoretical air-fuel ratio (stoichiometric) to lean.
The above-mentioned oxygen concentration detecting element must be maintained in an activating condition to keep the preserve the accuracy of the detected air-fuel ratio. Usually, by energizing a heater provided in the element after the engine is started, the element is heated and activated early. To keep that activating state, the electric power supplied to the heater is controlled.
FIG. 45 is a diagram showing a correlation between the temperature of the oxygen concentration detecting element and an impedance thereof. There is a correlation shown by a solid line in FIG. 45, that is, that the impedance of the element is attenuated with a rise of the element temperature. Paying attention to this relation, in the above described control of energization of the heater, feedback control is carried out so that an impedance of the element is detected to introduce an element temperature and that element temperature is adjusted to a desired activation temperature, for example, 700.degree. C. For example, when the impedance Zac of the element corresponding to the initial control element temperature 700.degree. C. is 30 .OMEGA. or more (Zac.gtoreq.30) as indicated by the solid line of FIG. 45 between the temperature of the oxygen concentration detecting element (hereinafter simply referred to as an element), that is, the element temperature is 700.degree. C. or less, electric power is supplied to the heater. If the Zac is smaller than 30 .OMEGA. (Zac&lt;30), or the element temperature exceeds 700.degree. C., the supply of electric power to the heater is released so as to maintain the temperature of the element more than 700.degree. C. thereby keeping the activating condition of the element. Further, when electric power is supplied to the heater, duty control is carried out so that an electric power amount necessary for eliminating a deviation (Zac-30) between an element impedance and its target value is obtained and that electric power amount is supplied.
For example, according to a related technology disclosed in Japanese Patent Application Laid-Open No. HEI 9-292364, when an impedance of the oxygen concentration detecting element is detected, an AC voltage of a preferred frequency is applied to detect an element temperature so as to detect the impedance. By applying the voltage of that frequency, a resistance of an electrolyte portion of the element can be measured. Because the resistance of the electrolyte portion does not change largely by aging, likewise the element impedance does not change largely. Therefore, it can be considered that the relation between the element temperature and impedance indicated by the bold line of FIG. 45 is substantially maintained unchanged irrespective of aging.
However, after the oxygen concentration detecting element has aged, a correlation between the element temperature and impedance is as shown by the dotted line of FIG. 45.
Here, a structure of the air-fuel ratio sensor, equivalent circuit and impedance characteristic will be described.
FIG. 46A is a sectional structure diagram of the air-fuel ratio sensor element and FIG. 46B is a partially enlarged diagram of the electrolyte portion.
FIG. 47 is a diagram showing an equivalent circuit of the air-fuel ratio sensor element. In FIG. 47, R1 denotes a bulk resistance of the electrolyte composed of, for example, zirconia (grain portion in FIG. 46); R2 denotes a granular resistance of the electrolyte (grain boundary portion of FIG. 46); R3 denotes an interface resistance of an electrode composed of, for example, platinum; C2 denotes a granular capacitive component of the electrolyte (any grain bound part in FIG. 46); C3 denotes a capacitive component of the electrode interface and Z(W) denotes an impedance (Warburg impedance) generated when the interface concentration changes periodically as electric polarization is carried out by the AC current.
FIG. 48 is a diagram showing an impedance characteristic of the air-fuel ratio sensor element. The abscissa indicates a real part Z' of the impedance Z and the ordinate indicates an imaginary part Z". An impedance Z of the air-fuel ratio sensor element is expressed by Z=Z'+jZ". From FIG. 48, it is evident that the electrode interface resistance R3 converges to 0 as the frequency approaches 1 to 10 kHz. Further, a curve indicated by a dotted line indicates an impedance which changes when the air-fuel ratio sensor element is deteriorated. From a portion of the impedance characteristic indicated by this dotted line, it is evident that particularly R3 changes by aging. When the oxygen concentration of gas detected by the air-fuel ratio sensor element changes rapidly also, the impedance characteristic changes as indicated by the dotted line.
FIG. 49 is a diagram showing a relation between the frequency of AC voltage applied to the air-fuel ratio sensor element and the element impedance. FIG. 49 is obtained by converting the axis of abscissa of FIG. 48 to frequency f and the axis of ordinate to impedance Zac. From FIG. 48, it is evident that the impedance Zac converges to a predetermined value (R1+R2) in 1-around 10 kHz-10 MHz in frequency and the impedance Zac decreases on a higher frequency than 10 MHz so that it converges to R1. Therefore, to detect the impedance Zac in a stabilized state, it is evident that the near 1-around 10 kHz-around 10 MHz in which the Zac is constant regardless of the frequency is desired. Further, the curve indicated by the dotted line indicates an impedance when an AC voltage of a measurable low frequency (1 kHz or less) is applied to the R3 which changes by aging. From the low frequency impedance, the degree of the deterioration of the air-fuel ratio sensor element is determined.
As indicated by the dotted line of FIG. 45, the correlation between the temperature of the oxygen concentration detecting element which is an air-fuel ratio sensor element and an impedance of 1-around 10 kHz-10 MHz changes largely after the element is deteriorated as compared to when it is new.
However, according to Japanese Patent Application Laid-Open No. HEI 9-292364, because only a portion corresponding to a resistance R1+R2 of the air-fuel ratio sensor is measured, the characteristic change of the air-fuel ration sensor element cannot be grasped. Therefore, if the control on energization of the heater is continued with the element impedance Zac as the element temperature control target value maintained at 30 .OMEGA., the control element temperature after the element is deteriorated increases gradually, so that, for example, it is set up to 800.degree. C. Therefore, there is a problem that the element is over heated so that the deterioration is accelerated, thereby the service life thereof being reduced.
When the AC voltage of the low frequency of 1-around 10 kHz is applied to the air-fuel ratio sensor as shown in FIGS. 48, 49, a detected low frequency impedance changes largely after the element is deteriorated as compared to when the element is new.
However, according to Japanese Patent Application Laid-Open No. HEI 9-292364, because only the portion corresponding to the resistance R1+R2 of the air-fuel ratio sensor element is measured, the characteristic change of the air-fuel ratio sensor element cannot be grasped. Therefore, the element temperature or element characteristic changes so that calculation of the air-fuel ratio from the output of the air-fuel ratio sensor becomes inaccurate, thereby worsening emission from the engine. Alternatively, because the failure of the air-fuel ratio sensor or activating condition is determined based on an element impedance detected when the element temperature or element characteristic is changing, there is produced a problem that accurate determination of these factors is disabled.